The diffractive lens is an important component in the micro-electromechanical systems and in the optical systems. The conventional diffractive lens has a so-called "terraced" diffractive structure which is prepared through etching or burning-off with laser beam, forming a multilevel structure on the substrate of the lens. Among the conventional approaches etching is frequently employed due to its low cost and convenience in operation.
In the conventional art, the diffractive lens is prepared under a multi-step etching process, by using a plurality of masks. FIGS. 1a to 1c show the steps of the conventional preparation of the terraced diffractive lens. FIG. 1a illustrates the structure of the first mask and the structure of the substrate after the first etching step. As shown in the figure, on the substrate 4, the area corresponding to the transparent area of the first mask 1 is developed and etched to form a two-step terrace structure. FIG. 1b illustrates the structure of the second mask and the structure of the substrate after the second etching step. As shown in the figure, on the substrate 4, the area corresponding to the transparent area of the second mask 2 is developed and etched to form a four-step terrace structure. FIG. 1c illustrates the structure of the third mask and the structure of the substrate after the third etching step. As shown in the figure, on the substrate 4, the area corresponding to the transparent area of the third mask 3 is developed and etched to form a eight-step terrace structure. By using the etching process as described above, it is possible to form a 2.sup.n -step terrace structure on the substrate 4 with n masks. When the material of the substrate is silicon or germanium, the etched substrate 4 can function as a diffractive lens for infrared.
In the conventional preparation of the diffractive lens, however, a plurality of masks and a series of etching (including developing) process are required. This does not only make the preparation of the diffractive lens time consuming and costly but that the problem of misalignment of the masks can not be avoided. For example, the distance between each stage of the terrace is approximately 2 .mu.m. When any two of the masks are not aligned, the lens so prepared would be defected. This misalignment problem has become the major obstacle in enhancing the yield rate of the diffractive lens.
It is thus a need in the industry to have a diffractive lens that can be prepared under one-step etching process and with one single mask. It is also a need to provide a simplified method for the preparation of diffractive lens.